1. Technical Field
The present invention relates to a development roller having a roughness on the circumference thereof for transporting toner to a latent image bearing unit, a development device containing the development roller, and an image forming apparatus containing the development device.
2. Related Art
Development devices developing a toner image from a latent image with one-component non-magnetic toner triboelectrically charge the toner on a development roller. The development roller has a surface roughness on the circumference thereof, the roughness having a substantially flat top surface. The development roller includes a base unit having a roughness portion of grooves on the circumference thereof formed through component rolling, and a surface layer having a uniform thickness formed through plating on the surface of the base unit (as disclosed in Japanese Unexamined Patent Application Publication No. JP-A-2007-121948).
As illustrated in FIG. 11A, a development roller a includes a base unit b and a surface layer c plated on the base unit a as a coverage.
A toner feed roller and a toner regulator unit typically remain in contact with the development roller. Silica having a high hardness is used as an external additive plating toner mother particles. As image forming operations are repeated by many times, the outer circumference of the development roller is worn. The surface layer on the development roller is thus designed to control wear of the circumference of the development roller.
When a roughness pattern is pressed against the base unit of a development roller a to form a roughness portion, the material of the base unit corresponding to the recess is swollen around the recess. Referring to FIG. 8A, a small portion e is swollen in a radially external direction of the development roller a (in an upward direction in FIG. 8A) from a main surface d serving as a regular surface of a base projection c at the side edges of the base projection c of a main unit b of the development roller a. If the roughness depth is large, the material amount of the recess is large, and the swollen portion e is pronounced. A small base recess f can be caused on the base surface d of the base projection c through centerless machining performed prior to component rolling.
When the surface layer g is formed on the base unit b, a plating process is performed with no consideration given to the base swollen portion e and the base recess f in the related art. A swollen portion i of the surface layer g is caused at the base swollen portion e having a height h from the base surface d larger than a thickness t of the surface layer g (t<h).
Although the wear of the surface layer g of the development roller a is controlled as previously described, the degree of wear of the surface layer increases in a long service life of the development roller a. As the surface layer g is worn, the swollen portion i of the surface layer g is rapidly worn as illustrated in FIG. 8B, and the base swollen portion e is exposed at an early stage of service. Even if the durability of the development roller a is increased with the surface layer g, the level of durability of the development roller a is not sufficient. There is still room for improvement in the durability of the development roller a.
Similarly, a base portion k may be swollen in a direction toward the base recess j at the side edge of the base projection c of the base unit b of the development roller a as illustrated in FIG. 8C. A swollen portion n of the surface layer g is caused at the base swollen portion k having an expansion h′ from a base side surface m as the regular surface larger than the thickness t of the surface layer g (t<h′). The durability of the development roller a is not sufficient, and there is room for improvement.
An amount of toner e transported by the development roller a is regulated by a toner regulator blade f. In one toner regulating method, a predetermined area f2 of the toner regulator blade f, including a front edge portion f1 illustrated in FIG. 11A or a front edge portion f2 illustrated in FIG. 11B, is slid on a flat portion h of a projection g so that the flat portion h has partially no toner e with most of the toner e held in a recess i.
Referring to FIG. 11C, a surface k of the base projection j of the base unit b has a large number of small dents m (only one dent m shown for convenience of explanation) through centerless machining performed prior to the machining of the roughness portion of the base unit b. For this reason, the surface of the base projection j has some degree of surface roughness. A surface layer c at the projection g covering the base projection j also has a large number of small dents n (only one dent n shown for convenience of explanation). The surface layer c has some degree of surface roughness.
In addition to the toner regulator blade f, a toner feed roller (not shown) is forced to be in contact with the development roller a. One of silica and titania, having a high hardness, is used as an external additive covering toner mother particles of the toner e. As the image forming operations are repeated by many times, the surface of the surface layer c at the flat portion h is worn because the toner feed roller and the toner regulator blade f press silica and titania against the development roller a. As illustrated in FIG. 11D, the surface of the surface layer c is smoothed (to a mirror state free from small roughness). A contact area of the toner regulator blade f to the projection g thus increases. As a result, the contact level between the development roller a and the toner regulator blade f is thus raised. The contact level tends to be even larger if part of the flat portion h of the projection g is not covered with the toner e, or if the toner regulator blade f is made of rubber.
If the contact level between the development roller a and the toner regulator blade f is high, the toner regulator blade f suffers from uneven sliding. The toner regulator blade f may be even broken at the front end thereof. A rasping sound may be caused when the toner regulating blade f is pressed against the development roller a. In view of a long service life, there is room for improvement in the durability of the development roller a and the contact level.